1. Filed of the Invention
The present invention relates to a pulse width modulation (PWM) controller of flyback power converter, and more particularly, to a synchronous rectifier PWM controller (SR-PWM) for a flyback power converter to improve efficiency of power conversion.
2. Description of Related Art
Power converters have been frequently used for converting an unregulated power source to a constant voltage source. Among various power converters, flyback topology is the most common one. A transformer having a primary winding and a secondary winding is the major part of a flyback power converter. In application, the primary winding is connected to the unregulated power source, preferably a DC voltage source, and a switching device is connected to the primary winding to switch on and off the connection between the unregulated power source and the primary winding. A rectifying diode is typically connected to the secondary winding for rectifying the energy transferred from the primary winding into a DC voltage.
FIG. 1 shows a conventional flyback power converter. The flyback power converter includes a transformer 10 having a primary winding NP and a secondary winding NS; a switching device 5 connected to the primary winding NP of the transformer 10; a rectifying diode 15 and an output capacitor 30 connected to the secondary winding NS of the transformer 10. The flyback power converter operates in a two-step cycle. In a first step, the switching device 5 is turned on to establish a connection between an input voltage source VIN and the primary winding NP. Meanwhile, as the rectifying diode 15 is reverse biased, the conduction path via the secondary winding NS is cut off, and the primary winding NP operates as an inductor to store energy. In a second step, the switching device 5 is turned off, such that the primary winding NP is disconnected from the power source VIN. Under such conditions, the energy stored in the transformer 10 is released through the secondary winding NS, and is stored into the output capacitor 30.
In the topology as shown in FIG. 1, when the energy is released through the second winding NS, a forward voltage drop across the rectifying diode 15 inevitably causes conduction loss and renders the rectifying diode 15 as the dominant loss component. To resolve the power loss problem, a MOSFET 20 having low on-resistance has been used to replace the rectifying diode 15 and provides synchronous rectification of the flyback power converter.
FIG. 2 shows a conventional flyback power converter having a MOSFET synchronous rectifier (SR) 20. Similarly to the topology as shown in FIG. 1, the flyback power converter includes a transformer 10, a switching device 5 controlling conduction status between the primary winding NP of the transformer 10 and an input voltage source VIN, and an output capacitor 30 connected to the secondary winding Ns of the transformer 10. Unlike the topology as shown in FIG. 1, the flyback power converter as shown in FIG. 2 includes the MOSFET synchronous rectifier 20 for reducing the rectifying loss.
A flyback power converter normally has two operation modes, i.e. discontinuous operation mode and continuous operation mode. In the discontinuous operation mode, all the energy stored in the transformer is completely delivered before the next cycle starts. Therefore, no inducted voltage will remain in the transformer to resist the output capacitor discharging back to the transformer. As shown in FIG. 2, when the flyback power converter operates under the discontinuous operation mode, at the switching instant that the energy of the transformer 10 is completely delivered, a reverse current will be discharged from the output capacitor 30.
In a first operation phase, the switching device 5 is turned on to conduct the input voltage source VIN to the primary winding NP, and energy is stored to the transformer 10. The energy ε stored in the transformer 10 can be expressed as:ε=LP×IP2/2,where LP is the inductance of the primary winding NP, and IP is the current flowing through the primary winding NP. In the discontinuous mode, IP can be expressed by:IP=VIN×TON/LP,where TON is the duration when the switching device 5 is turned on. Therefore, the energy ε can be expressed as:ε=VIN2×TON2/2LP.
In a second operation phase, the connection between the primary winding NP of the transformer 10 and the input voltage source VIN will be cut off and the energy stored in the transformer 10 will be freewheeled to the output capacitor 30. The flyback power converter operates in the discontinuous mode under light load conditions, under which the energy stored in the transformer 10 is completely released before the next switching cycle starts. By completely releasing the energy stored in the transformer 10, no inducted voltage will remain in the transformer 10 to resist the output capacitor 30 discharging back to the transformer 10. Therefore, at the instant that the switching device 5 is turned off, a current will be discharged from the output capacitor 30 in a reverse direction once the energy stored in the transformer 10 is completely released.
In contrast, in the continuous operation mode, some energy remains in the transformer 10; that is, before the current released from the secondary winding Ns drops to zero, the next switching cycle will start. When the MOSFET synchronous rectifier 20 is switched off after the start of the next switching cycle, as shown in FIG. 3, a reverse charging operation of the output capacitor 30 may occur. More specifically, in the continuous mode, the energy ε stored in the transformer 10 can be expressed as:ε=[VIN2×TON2/(2×LP)]+[Ia×VIN×TON/T],where Ia is a current representing the energy that still exists in the transformer 10 when the next switching cycle starts; and T is the switching period of the flyback power converter.
Under the continuous mode operation, the transformer 10 keeps freewheeling the energy when the next switching cycle starts. If the MOSFET synchronous rectifier 20 is not switched off before the next switching cycle starts, the output capacitor 30 will be charged in a reverse direction.
Many approaches of synchronous rectification have been proposed to reduce rectifying loss, for example, U.S. Pat. No. 6,400,583, “Flyback converter with synchronous rectifying” issued to Chi-Sang Lau on Jun. 4, 2002 and U.S. Pat. No. 6,442,048, “Flyback converter with synchronous rectifying function” issued to Xiaodong Sun and John Xiaojian Zhao on Aug. 27, 2002.
However, in the disclosures mentioned above, the output capacitor is still sharply charged and discharged via the MOSFET synchronous rectifier at the switching instant in both continuous mode and discontinuous mode. Therefore, the efficiency is reduced and the noise is increased. Furthermore, in the above approaches, the transformer requires an additional auxiliary winding to generate a driving signal to achieve synchronous rectification; and thus increases the complexity thereof.